It is inherent in an engine and transmission system that changing the ratio of the transmission while the engine is operating causes the engine to change its speed, the magnitude of the change depending in part on other variables such as the position of the engine throttle. The present invention is particularly concerned with infinitely variable transmission systems which are controlled during operation of the motor vehicle in a manner such that the engine will operate at a desired speed, to the extent permitted by the range of ratios available for the particular transmission. A transmission system of this type is disclosed in British Pat. No. 1,525,674, and the present invention is an improvement over the general type disclosed therein.
The infinitely variable transmission described in British Pat. No. 1,525,674 is of the kind which includes an endless belt or chain running over a primary and a secondary V-shaped pulley, each pulley being constructed of two conical discs, one conical disc of the primary pulley being fixed to the transmission input shaft and one conical disc of the second pulley being fixed to the transmission output shaft. The other conical disc of each pulley is axially adjustable toward and away from the respective shaft-connected disc so that the running diameters of the pulleys can be changed in order to change the ratio of the transmission. Adjustment of the two axial positions of the axially movable discs is effected simultaneously by hydraulic means in a manner such that movement of the disc of the secondary pulley compensates for movement of the primary pulley disc. The control device for the pulleys includes a hydraulic spool valve which controls the supply and exhaust of pressure fluid to the hydraulic actuator for the axially movable disc of the primary pulley. The axially movable spool of the valve is biased in one direction by a fluid pressure which, through a Pitot tube, is built up in proportion to the speed of the transmission input shaft, i.e. in proportion to engine speed. The spool is biased in the opposite direction by a compression spring which in turn is biased in proportion to, for example engine intake manifold pressure or the position of the engine throttle. The control system operates in a mode such that the transmission ratio of the transmission is adjusted so that the engine speed assumes a given value, depending e.g. on the position of the throttle and, of course, to the extent permitted by the range of ratios available within the particular transmission.
In order to improve the driveability, in particular the reaction of a vehicle with an infinitely variable transmission considerable research has been carried out and various proposals have been made, such as the incorporation of certain delays in the control system and the influencing the transmission ratio by motor torque or tractive force.